The invention relates to a method and device for detecting the flow rate of a flowing medium, in particular for detecting the air flow prevailing in the intake tube of an internal combustion engine and therefore determining the aspirated air mass.
In order to detect the air mass aspirated by an internal combustion engine, usually a hot film air mass measuring device is used. These devices have a heatable element, which is exposed to the air flow to be measured and is cooled by it. There are a variety of possibilities for embodying the hot film air mass measuring device, both for the heating regulation and the evaluation method. Both types of air mass measuring devices and an existing method for detecting the air mass flow with a hot film air mass measuring device are based on the measurement of the heat imparted to the air mass flow that is flowing past it. Air mass measuring devices of one type measure the electrical energy required for regulating the hot film to a constant temperature. A second method and an associated second sensor device are also based on the fact that the hot film is regulated to a constant temperature. However, the measurement signal used here is not the required calorific output, but the temperature profile at the edge of the hot film that is embodied as a membrane. With the aid of a temperature sensor disposed upstream of the heating region and a temperature sensor disposed downstream of it, the temperature difference between these two points is determined. Both of the temperature sensors, which are embodied as temperature-dependent resistances, are components of a bridge circuit. A measurement signal is obtained from the bridge voltage produced and represents the temperature difference between the temperature-dependent resistance upstream of the heating region and the temperature-dependent resistance downstream of the heating region.
Both types of sensors and evaluation methods can be impaired by interference effects, for example airborne moisture or dirt. This can cause a sensor of this kind to produce a false reading or can lead to an error in signal evaluation.